1. Field of the Invention
The present invention relates to a high-frequency circuit element including resonators as basic elements. Prominent examples of such a high-frequency circuit element are filters and duplexers used for high-frequency signal processors in communication systems.
2. Description of the Prior Art
In high-frequency communication systems, high-frequency circuit elements based on resonators, for which filters and duplexers are prominent examples, are indispensable elements. Especially in mobile communication systems, the efficient utilization of frequency bands requires filters with narrow passbands. In base stations for mobile communications systems and communication satellites, there is a great need for filters with low loss, that are small and that can withstand high levels of power.
The majority of high-frequency circuit elements, such as resonator filters that are presently in use includes, for example, elements using dielectric resonators, elements using transmission line structures, and elements using surface acoustic wave elements. Of these, elements using transmission line structures are small and can be used for frequencies up to millimeter waves and microwaves. They are widely used, because they are of two-dimensional structure and formed on a substrate, so that they easily can be combined with other circuits and elements. For these types of resonators, half-wavelength resonators based on transmission lines are most widely used, and high-frequency circuit elements such as filters can be obtained by coupling several half-wavelength resonators together.
Another conventional example is a planar circuit structure. Typical examples of such a structure are (i) chains of circular disk resonators and/or (ii) elements achieving filter characteristics by providing protrusions in a part of the periphery of a circular disk resonator and thereby coupling dipole modes (see for example "Low Loss Multiplexers with Planar Dual Mode HTS Resonators", by Jerry Fiedziuszko et.al., IEEE Transactions on Microwave Theory and Techniques, Vol. 44, No. 7, pp. 1248-1257; article in IEICE Technical Digest, 1993, Vol. 93, No. 363 (XCE93 47-56) by Yasunari Nagai; "Analysis of Microwave Planar Circuit", IEICE Technical Digest, 72/8 Vol. 55-B No. 8, Tanroku MIYOSHI and Takanori OKOSHI).
However, in resonators with the transmission line structure using, for example, half-wavelength resonators, high-frequency current concentrates partially in the conductor, so that the losses due to the resistance of the conductor are comparatively large, and the Q factor of the resonator deteriorates, which causes an increase of losses in the case of a filter. Furthermore, in the case of half-wavelength resonators with microstrip transmission line structure that are frequently used, there is the problem of losses due to radiation from the circuit into space.
These factors become even more conspicuous as the structure is miniaturized and the operation frequencies are increased. As resonators with comparatively low losses and high power handling capability, dielectric resonators are used. However, since dielectric resonators have a three-dimensional structure and are relatively large, it is difficult to miniaturize high-frequency circuit elements using them.
Using superconductors, it is possible to reduce losses in the high-frequency circuit element. However, in the above-noted conventional structures, superconductivity is easily lost by excessive current concentrations, and it is difficult to use superconductors for signals with high power. In actual measurements, the largest input power was on the order of 10 mW, which is far from practical levels.
In filters using planar circuit resonators, for which circular disk resonators are prominent examples, the current distribution becomes uniform over a large area, so that they have excellent power handling capability. However, elements where several circular disk resonators are lined up in a row have a very large surface area so that it becomes very difficult to design multistage structures to attain steep skirt. Moreover, in the case of resonator filters using a planar circuit structure with protrusions in a portion of the periphery, there has been, so far, no easy method of designing multistage structures of three or more stages.
Therefore, in order to obtain a two-dimensional high-frequency circuit element that can be matched well with other circuit elements in the microwave and millimeter wave range, using high-performance yet small resonator filters, it is very important to solve the above-noted problems for resonators of transmission line structure or planar circuit structure.